Electron Acceleration in a Plasma Wakefield Accelerator E200 FACET, SLAC

Transcription

1 Electron Acceleration in a Plasma Wakefield Accelerator E200 FACET, SLAC Chan Joshi UCLA Making Big Science Small : Moving Toward a TeV Accelerator Using Plasmas Work Supported by DOE

2 Compact and Cheaper High-Energy Colliders a Grand Challenge for Science and Engineering in the 21 st century Particle Physics Project Prioritization Panel (P5) Report 2014: Building for Discovery A primary goal, therefore, is the ability to build the future generation accelerators at dramatically lower cost. For e + e - colliders, the primary goals are improving the accelerating gradient and lowering the power consumption NAE Grand Challenges for Engineering Engineer Tools of Scientific Discovery..engineers will be able to devise smaller, cheaper but more powerful atom smashers, enabling physicists to explore realms beyond the reach of current technology.

3 UCLA Advanced Accelerator R&D VISION Transformational R&D for a TeV scale e+e- collider To address critical physics issues for realizing an accelerator based on advanced concepts at the energy frontier in the next decade. A by- product will be compact accelerators for industry & science UCLA vision is well matched to P5 and NAE priorities for long range Accelerator R&D

4 Beam-Driven Plasma Wakefield Accelerators (Blowout Regime n b > n p ) Rosenzweig et W. Lu et al PRL 2006 Pukhov and Meyer-te-vehn 2002 (Bubble) Space charge of the beam displaces plasma electrons Plasma ion channel exerts restoring force => wake oscillation Linear focusing force on beams (F/r=2pne 2 /m) Accelerating field independent of r. No phase slippage M.Hogan et al, PRL 2005, P.Muggli et al PRL 2004 NJP 10, CERN Courier 10

5 PWFA Experiments Carried out at SLAC since 2000 At FACET, 2km of SLAC linac provides 50fs, 3 nc, 20 GeV e -, e + pulses at 1-10 Hz San Francisco Bay Stanford When focussed to a few microns I > W/cm 2, P > 200TW or W = 10J in 50fs Very reliable &Comparable to highest power lasers 5

6 Experimental Setup 20GeV 3 nc < 30µm Plasma is either self-ionized or pre-ionized by a axicon over up to 2 meter using a 250 mj, 100 fs Ti-sapphire laser E200 Collaboration

7 Beam-driven Plasma Wakefield Acceleration Length Scaling of Energy gain Loss Gain PLASMA LENGTH (cm) M. Hogan et al Phys. Rev. Lett. (2005) P. Muggli NJP(2010)

8 Beam-Driven Wakefield Acceleration from 42 GeV-85 GeV in 85 cm. V 445 p741 (2007) Experiment Simulations 35 Energy (GeV) 100 Gradient 50 GeV/m over a meter I. Blumenfeld et al Nature 2007 Plasma Accelerators will be compact but will they be efficient??

9 The FACET E200: PWFA Collaboration Goal: Accelerate a narrow energy spread bunch of electrons and positrons containing sufficient Charge so as to extract a significant fraction of energy from the wake E. Adli, J. Allen, W. An, C.I. Clarke, C.E. Clayton, S. Corde, J.P. Delahaye, A.S. Fisher, J. Frederico, S. Gessner, S.Z. Green, M.J. Hogan, C. Joshi, M. Litos, W. Lu, K.A. Marsh, W.B. Mori, P. Muggli, N. Vafaei-Najafabadi, D. Walz, V. Yakimenko Work supported by DOE contracts DE-AC02-76SF00515, DE-AC , DE-FG02-92-ER40727 and NSF contract PHY

10 SIMULATION δp/p (%) EXPERIMENT: RF STREAK CAMERA We Inject a Separate bunch with Sufficient Charge TRAILING SCATTERING FOIL DRIVE SCATTERING FOIL IN 100 fs 500 fs TRAILING DRIVE Peak beam current no longer enough to Ionize Li, so need a pre-ionized plasma Impose a positive chirp Disperse the beam Place appropriate masks Recompress the beam N drive = 6.0e9 ~ (1nC) N trailing = 2.0e9 ~(0.3 nc) Small (O(0.1 0 ) changes in the phase ramp leads to beams spectrum and Therefore changes to trailing/drive charge

11 Beam Loading: Key to Small Energy Spread and High Energy Extraction Drive Trailing E + E - For a given drive bunch charge Weiming An et al PRSTAB 2013 T = E + /E- reduces as trailing charge increases, But E + flattens as the wake is strongly loaded therefore efficiency expected to increase 11

12 E (GeV) Loss Gain E (GeV) E (GeV) Loss Gain E (GeV) Loss Gain Acceleration of a Discrete Bunch of Electrons Use a 30 cm long preformed Li-plasma 90 pc in core of trailing bunch Same amount of charge accelerated outside core Core energy gain: 1.7 GeV E 0 Spectrally dispersed final beam trailing bunch drive bunch E 0 Imaged Energy Setting: 22 GeV L o s s G ai n Core energy spread E 0 E 0 < 2% Gradient of ~5 GeV/m Laser Off: No Plasma Interaction 12

13 Comparison to Simulation Particle-In-Cell (PIC) simulation with QuickPIC (UCLA) for beam-plasma interaction PIC output then propagated through simulated beamline Shows very good qualitative agreement with observed final spectrum Gives insight into beam-plasma coupling: trailing bunch was too long and wide to fully couple into plasma wake Shows loading of wake key to efficient energy extraction Core = z - ct PIC Simulation Final Dispersed Beam Profile 13

14 Evidence for Beam Loading Efficiency Variation is correlated to Trailing Bunch/ Drive Bunch charge ratio Total Efficiency Core Efficiency Core For a given drive bunch charge as trailing charge increases E + flattens as the wake is strongly loaded Therefore efficiency expected to increase M. Litos et al, Nature 4 th Nov,

15 Energy Spread of the Accelerated Bunch Median energy spread of 350 MeV or 1.7% Initial Energy Spread on the beam 1% The increase in energy spread expected from non-optimal beam loading of wake Intrinsic Energy Spread 1.5% ΔE/E 15

16 Efficiency versus Energy Spread Smallest energy spread is on the order the initial energy spread. This implies wake flattening due to near optimum beam loading 16

17 Energy (GeV) Increased Plasma Length Increased Energy Gain Shots ordered by drivewitness bunch separation Single shot with 6 GeV Energy Gain 26 GeV 2014: increased plasma length from 30cm to 130cm Increased energy gain Reduced plasma density to 26 5x10 16 cm -3 for better coupling 24 Early analysis: ~100pC accelerated O(10%) energy spread 20 smaller separation mean energy gain 6 GeV 17

18 Summary of Electron Acceleration 1) High Efficiency, high-gradient acceleration of a distinct beam shown 2) The beam contains a significant amount of charge and has a narrow final energy spread Accelerated beam and Wake Parameters are as follows: Charge in core pc Energy spread 1-10% Initial energy spread 0.5% Total Energy extraction efficiency Up to 50% Core beam energy extraction efficiency Up to 30% Energy Gain 1.7 GeV (36 cm ), 6 GeV (1.3m) Average Gradient 5 GeV/m Unloaded Transformer Ratio 2 Beam Loaded Transformer Ratio 1

19 Conclusions Tremendous progress on second generation electron acceleration in a PWFA at FACET Acceleration of a significant charge Small energy spread High gradients High energy transfer efficiency per unit length Next Great Challenge is ultra-low emittance beams